The present invention represents an improvement over the phase shifters described in commonly owned U.S. patent application Ser. No. 10/290,838 entitled “Variable Power Divider” filed on Nov. 8, 2002 and U.S. patent application Ser. No. 10/226,641 entitle “Microstrip Phase Shifter” filed on Aug. 23, 2002, which are incorporated herein by reference. The relevant background technology described in those applications will not be repeated here. In addition, the phase shifter described in this specification may be deployed in the dual-polarization antenna described in commonly owned U.S. patent application Ser. No. 10/623,379 entitled “Vertical Electrical Downtilt Antenna” filed on Jul. 18, 2003, which is also incorporated herein by reference. Again, the background technology relevant to this embodiment of the invention is described in that application and will not be repeated here.
Generally, the market for wireless base station antennas is highly price and performance competitive. Therefore, there is an on-going need for cost effective techniques for providing the technical features desired for these antennas. For example, advancements that reduce the size, cost, complexity, or number of moving parts are generally desirable. Of course, accurate and repeatable performance, as well ruggedness, longevity and low maintenance costs are also desirable. Meeting these competing design objectives is particularly challenging with respect to the moving parts of the antenna, such as the phase shifters used for beam steering and in variable power dividers, which may also be used for beam steering.
In particular, conventional phase shifters have used a wiper arm that slides along a transmission media trace located on a backplane to implement a differential phase shifter. See, for example, Japanese publication number 06-326501, published 25 Nov. 1994, naming Mita Masaki and Tako Noriyuki as inventors. This type of phase shifter can experience failure if the wiper arm loses electrical communication with the transmission media trace. Because wireless base station antennas are typically deployed outdoors on buildings or towers, they are subject to the variable stresses and dimensional changes induced by temperature changes, vibration and external forces of wind, and other types of environmental conditions and variations over extended periods of time. These conditions can cause relative dimensional changes to occur between the components of the phase shifter assembly that can result in changes in the degree of wiper contact with the transmission media trace. Changes in wiper contact, such as partial wiper arm separation, can result in operational performance changes of the antenna. In extreme cases, complete wiper arm separation can result in operational failure of the antenna.
One conventional approach to solving the wiper arm separation problem is shown in FIG. 1. This configuration includes a slot 1 through the backplane 2 adjacent to the transmission media trace 5 and a spring-loaded set screw 3 extending from the wiper arm 4 through the slot. This approach is very effective at maintaining electrical communication between the wiper arm 4 and the transmission media trace 5, but has the disadvantage of requiring a slot through the backplane 2. This is a problem because in a typically wireless base station antenna, the backplane serves as an exterior wall intended to keep out the weather elements. Cutting slots through the backplane can cause water to enter the antenna, which can cause the antenna to short, corrode, and freeze if the temperature drops. To solve this problem, the phase shifter shown in FIG. 1 does not use the backplane 2 as an exterior enclosure wall, but instead houses the backplane in an enclosure 6 that includes a separate exterior wall 7. Providing this exterior wall in addition to backplane 2, as well as brackets for supporting the backplane within the enclosure 6, increases the cost and complexity of the antenna.
In addition, dual-polarization antennas typically include a duplication of actuator, transmission and radiating elements; one for each polarization. Outfitting dual-polarization antennas with beam steering phase shifters in the conventional manner likewise requires a duplication of the phase shifters and associated actuators. This type of duplication can be costly, particularly when the phase shifters are motor driven, which is desirable for remotely controlled operation. It is often desired to vary the phase in a like manner for each polarization to achieve corresponding characteristics. For this reason, commonly operating the phase shifters in a coordinated manner advantageously eliminates duplicate components.
Accordingly, there is an ongoing need for more cost effective systems for implementing phase shifters for wireless base station antennas including dual-polarization antennas. There is a further need for phase shifters for dual-polarization antennas that eliminate the duplication of parts.